1. Field of the Invention PA1 2. Description of the Prior Art
This invention relates to a technically simplified process for the production of cold-setting polyurethane urea elastomers in which high-melting aromatic diamines present in solid phase are reacted with isocyanate prepolymers in a heterogeneous reaction.
The production of polyurethane urea elastomers from polyisocyanates, relatively high molecular weight polyhydroxyl compounds and aromatic diamines is known. To guarantee reasonable processing times for reactive systems of the starting components mentioned, the most widely used reactive aromatic isocyanates are generally reacted with sluggishly reacting diamines. The diamines which have been successfully used in practice for this purpose include aromatic diamines in which the basicity and, thus, reactivity to isocyanates has been reduced by the introduction of halogen or carboxy substituents. 3,3'-dichloro-4,4'-diaminodiphenylmethane (MOCA), previously the most widely used aromatic diamine, is mentioned as an example.
U.S. Pat. No. 3,891,606 describes the crosslinking of NCO prepolymers of polyhydroxyl compounds and an excess of polyisocyanates with aromatic diamines in which the reactivity to isocyanate groups is reduced by complexing with certain alkali metal salts. The disadvantage of this process lies in the fact that it is confined to two particular aromatic diamines. In addition, the complex between the aromatic diamine and the alkali metal salt has to be prepared in a separate process step.
Another way of controlling the reaction velocity between polyisocyanatets and aromatic diamines is to carry out the reaction in an organic solvent. Corresponding processes are described, for example, in U.S. Pat. No. 3,926,922 and in published Japanese patent application 9195/70. The disadvantage of using organic solvents is obvious. First, the risk of fire and explosions is increased and, second, technically elaborate recovery of the solvent is necessary on economic and ecological grounds.
Previously, little has been known of the production of polyurethane ureas by the reaction of polyisocyanates with aromatic diamines in a heterogeneous phase. According to the prior art, the relatively high-melting aromatic diamines of commercial interest are either used in dissolved form, which involves the disadvantages just described, or are reacted with polyisocyanates in the melt. The processing of aromatic diamines in the melt is described, for example, in the above-cited U.S. Pat. No. 3,926,922 and in DE-AS 1,122,699.
DE-AS 1,122,699 relates to a process for the production of polyurethane elastomers by crosslinking liquid isocyanate prepolymers by reaction with mixtures of primary diamines and compounds containing several hydroxyl groups in molds, in which a dispersion of a powder-form crystalline diamine in a liquid polyester or polyether containing several hydroxyl groups or in castor oil is introduced into the prepolymer at a temperature below the melting point of the diamine and the resulting mass is hardened in known manner at temperatures above the melting point of the diamine in the mixture. In this process, therefore, the actual "amine crosslinking" again takes place in a liquid homogenous phase. The disadvantage of the process according to DE-AS 1,122,699 lies in particular in the high temperatures required for the processing of high-melting diamines such as 1,5-naphthylene diamine (Mp. 189.degree. C.) or 4,4'-diaminodiphenyl ether (Mp. 186.degree. C.) because experience has shown that decomposition reactions occur to a considerable extent in the polyurethane at these temperatures, uncontrollably altering the mechanical properties of the end products.
U.S. Pat. No. 3,105,062 describes a process for the production of polyurethane ureas in which relatively high molecular weight preadducts containing isocyanate groups are preferably reacted with aromatic diamines in a heterogeneous phase. The reaction mixtures harden at a temperature which the "2-phase system" changes into a "1-phase system". This temperature is generally in the range from 100.degree. to 170.degree. C.
However, the aromatic diamines mentioned in U.S. Pat. No. 3,105,062 are soluble, albeit to a limited extent, in the reaction medium (NCO preadduct) so that uncontrollable preliminary reactions occur during the mixing of the two components, even at room temperature. The result is that the reaction mixtures thicken up in a very short time, paste-like formulations being obtained in some cases. Such formulations are difficult to process by the usual casting technique and, accordingly, have to be brought into the required shape by application of pressure before they are actually hardened by application of heat. According to U.S. Pat. No. 3,105,062, the stability of the thickened reaction mixtures in storage (pot life) is sufficient for further processing (molding under pressure, coating) and amounts to several hours. It can be seen form the examples provided that the reaction mixtures in question are preferably reaction mixtures which have a maximum pot life of about 1 hour. Accordingly, they cannot be regarded as long-term stable systems.
In U.S. Pat. No. 3,105,062, it is specifically pointed out that the use of the diamines mentioned, which are present in solid form only, in a one-shot process gives unsatisfactory polyurethane moldings. In this case, the unwanted preliminary reaction of the diamine with the diisocyanate takes place to a greater extent such that the poorly soluble polyurea precipitates in the reaction mixture and ceases to react.
DE-OS 2,635,400 describes another process for the production of polyurethane urea elastomers, in which aromatic diamines are reacted as chain-extending agents in a one-shot or multistep process. This process is characterized in that the aromatic diamines are present in solid form in the reaction mixtures and have a melting point about 130.degree. C. The thermosetting of these mixtures takes place at a temperature in the range from 80.degree. to 120.degree. C., i.e. below the melting point of the aromatic diamine. By virtue of the choice of these diamines as chain-extending agents, there is no mixture-thickening preliminary reaction with the NCO preadduct (NCO prepolymer). Accordingly, systems such as these may also be readily processed by casting. Since the pot life of these reactive systems is considerably increased, it is possible in this process to use many aromatic diamines which have been very difficult to process by previously known techniques.
It can be seen from the examples of DE-OS 2,635,400 that the pot life of the liquid reaction mixtures varies from a few minutes to several hours in a certain temperature range, depending on the reactivity or solubility of the aromatic diamine. Under standard processing conditions, for example in hand casting, these reaction mixtures, particularly when they have relatively long pot lives, can generally be processed without major difficulties. By contrast, processing becomes difficult when, as a result of machine stoppages or other enforced interruptions, there is a relatively long interval between preparation of the reaction mixtures and the hardening phase. Accordingly, the need for a long processing time at low temperature and for a short setting time at elevated temperature is becoming increasingly more urgent in practice.
The final PUR plastics are generally required to show good mechanical properties and, in many cases, a thermal stability adapted to the application envisaged. In the prior art, the thermal stability of PUR elastomers depends largely on the type of chain-extending agent used. When glycolic chain extenders for example are used for the production of elastomers, the PUR moldings obtained show lower thermal stability than when compounds containing the amino groups are used. There are of course also distinct differences in regard to thermal stability within the particular type of chain-extending agents (either compounds containing OH or NH.sub.2 groups).
Many diamines varying in their constitution are mentioned in DE-OS 2,635,400 as suitable chain extenders for the production of polyurethane urea elastomers. 2,2'-diaminodiphenylurea is mentioned as the only representative of a diaminodiphenylurea and no example is provided.
Investigation of the application has shown that the reaction of an NCO preadduct with the diaminodiphenylurea mentioned above gives an elastic PUR molding characterized by entirely acceptable mechanical properties. However, the thermal stability of these elastomers is unexpectedly poor.
Even when the molding is conditioned at 120.degree. to 130.degree. C., there is a significant reduction in its mechanical properties after only a short time. At 140.degree.-150.degree. C., only a viscous melt is present, irrespective of whether the sample is hot or cold.
Two-component polyurethane urea casting systems (PUR casting systems) which react at room temperature or moderately elevated temperature, which may be formulated to have a pot life (casting time) of any length and which harden rapidly (tack-free and scuff-resistant, surface-dry) after the hardening process has started, are already known. The previously described "cold-setting" systems of long pot life all have the disadvantage that either the casting time is no longer than 20 or 30 minutes or the final solidification takes several days.
Accordingly, an object of the present invention is to provide a two-component PUR casting system which comes as close as possible to the ideal cold-hardening two-component PUR casting system, i.e., one having a long or adjustable pot life at room temperature and a short setting time.
More particularly, therefore, an object of the present invention is to provide a process for the production of polyurethane ureas in which the processing time of the reaction mixtures is at least several hours, preferably at least 8 hours (long-term stable system). It is also an object that the liquid reaction mixtures be able to be set at an economical rat at low reaction temperatures.
Another object of the invention is to provide suitable chain-extending agents containing amino groups and also suitable starting components (NCO preadducts). These NCO preadducts should only dissolve the solid chain extender minimally at room temperature without the polyadducts formed in small quantities leading to clumping or any significant increase in the viscosity of the mixture. The solid chain extender is required to change into the dissolved reactive form very slowly and continuously at room temperature. Because the reaction between the two reactants takes place in heavily diluted medium, the polyurethane urea is synthesized very slowly. On the other hand, however, the system should harden to its final state at an economical rate.
A final object of the present invention is to provide a process for the production of polyurethane urea elastomers in which high-quality elastomers having high thermal stability are obtained.
It has now surprisingly been found that finely divided diaminodiphenyl ureas corresponding to the formula ##STR2## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 may be the same or different and represent hydrogen radicals and wherein the NH.sub.2 groups are in the m- and/or p-position to the urea group, provide reaction mixtures which are stable in storage for long periods in combination with NCO preadducts.